1. Field of the Invention
The present invention relates to a superconducting wire, and more particularly, it relates to a superconducting wire which is prepared from an oxide superconductor.
2. Description of the Background Art
It has recently been reported that a composite oxide sintered body superconducts at a high critical temperature, and an attempt has been made to develop a superconductivity technique through such a superconductor. It has been reported that a YBaCuO oxide exhibits superconductivity at 90 K and BiPbSrCaCuO oxide exhibits superconductivity at 110 K.
Such an oxide superconductor exhibits superconductivity in liquid nitrogen, which can be easily obtained at a relatively low cost, and hence the same is expected for practical use. In order to apply the oxide superconductor to the wire of a superconducting magnet, for example, it is necessary to work the same into a wire. However, it is known that such an oxide superconductor is anisotropic, and hence the crystals thereof must be oriented in a specific direction in order to attain high critical current density (JC) In general, powder of the oxide superconductor is charged in a metal sheath, drawn into a wire having a prescribed diameter, and thereafter worked by rolling or pressing to obtain a tape-type wire, in which crystals of the oxide superconductor are oriented in a prescribed direction.
However, the oxide superconductor has such anisotropy that its critical current density is extremely varied with the direction of an applied magnetic field. Thus, it is known that critical current density is significantly varied with a magnetic field which is applied in parallel to the tape surface and that applied perpendicularly to the tape surface in a tape-type wire of an oxide superconductor whose crystals are oriented in a prescribed direction. For example, the wire exhibits critical current density of about 1000 A/cm2 when a magnetic field of 1 T is applied in parallel to the tape surface, while the current hardly flows in the case of a perpendicular magnetic field. When such a conventional tape-type wire is coiled to provide a superconducting magnet, the generated magnetic field is applied in different directions depending on a location of a wire portion in the coil, to cause difference in critical current density within the wire. The critical current density of the overall wire is dominated by portions having lower critical current density. Thus, it is impossible to attain high critical current density as a whole.
It is known that a magnetic flux enters the interior of an oxide superconductor before the superconductor goes into an ordinary conducting state. This magnetic flux is unmovably fixed by xe2x80x9cpinningxe2x80x9d, and it is necessary to improve such pinning force in order to attain high critical current density.
However, no contrivance has been made to improve the pinning force in the prior art oxide superconducting wire, and hence it has been impossible to attain high critical density.
A principal object of the present invention is to provide an oxide superconducting wire which can regularly keep constant critical current density regardless of the direction of application of a magnetic field, and a method of producing the same.
Another object of the present invention is to provide a superconducting wire having higher critical current density by improving pinning force.
The present invention provides a wire of an anisotropic oxide superconductor, which comprises a core part of the wire and a superconducting layer enclosing the core part so that specific crystal axes of the oxide superconductor are oriented toward the core part.
According to the present invention, the anisotropic oxide superconductor is not particularly restricted but may be prepared from a BiPbSrCaCuO oxide or a YBaCuO oxide, for example. The present invention is applicable to most of oxide superconductors since most of oxide superconducting wires are considered anisotropic in view of current flowability etc.
The inventive method of producing a superconducting wire comprises a step of arranging a metal sheath around a metal rod for serving as a core part of the wire and charging powder of an oxide superconductor in a clearance between the metal sheath and the metal rod to obtain a composite material, and a step of plastic deformation of the composite material so that the metal sheath is larger in reduction of area than the metal rod.
In order to plastically work the composite material so that the metal sheath is larger in reduction of area than the metal rod, the metal sheath and the metal rod are prepared from materials which are different in workability from each other, for example. In other words, the metal rod is prepared from an relatively unworkable metal, and the metal sheath is prepared from a relatively workable metal. Due to such selection of the materials, it is possible to make higher reduction of area of the metal rod while making lower that of the metal sheath. When the oxide superconductor is prepared from a BiPbSrCaCuO oxide, for example, the metal sheath is formed of Ag, which is unreactive with the oxide superconductor, and the metal rod is formed of a metal such as Ni, for example, which is inferior in workability of Ag. In this case, the surface of the metal rod is preferably covered with Ag or the like, so that Ni is not reactively in contact with the oxide superconductor.
The metal rod and the metal sheath may be prepared from the same metal while devising the method of plastic working so that the metal sheath is larger in reduction of area than the metal rod.
The plastic working method is not particularly restricted in the present invention, while wire drawing, rolling or swaging may be employed.
According to the present invention, the superconducting wire may have one or more core parts. A plurality of core parts may be provided in multi-core structure such as that of a conventional metallic superconducting wire.
According to the inventive oxide superconducting wire, a superconducting layer is provided around the core part so that specific crystal axes of the oxide superconductor are oriented toward the core part. Therefore, the superconducting layer contains such crystals of the oxide superconductor that specific crystal axes are oriented in directions which are different by 360xc2x0 from each other. Thus, any portion of the superconducting layer necessarily contains crystals of the oxide superconductor which exhibit the highest critical current density with respect to the applied magnetic field. In the inventive oxide superconducting wire, therefore, portions having high critical current density regularly exist in series along the longitudinal direction of the wire even if the magnetic field is partially applied in different directions. Thus, high critical current density can be maintained as a whole.
According to the inventive method, the composite material is so plastically worked that the metal sheath is higher in reduction of area than the metal rod. Due to such plastic working, a clearance between the metal sheath and the metal rod is gradually reduced in thickness to compress the powder of the oxide superconductor which is charged in this clearance. Through such compression, the powder of the oxide superconductor is subjected to force which is similar to that for the conventional tape-type wire, and oriented in a prescribed direction. For example, an oxide superconductor which is prepared from a BiPbSrCaCuO oxide is cloven along the c-plane, and hence the c-axes, which are perpendicular to the c-plane, are oriented toward the core part. The BiPbSrCaCuO oxide superconductor has the highest critical current density along the direction of the c-plane, and the wire is so produced as to orient the oxide superconductor in this direction. As hereinabove described, the superconducting layer is so formed as to orient the c-axes toward the core part in the radial section of the wire, which is necessarily provided with portions having the highest critical current density with respect to any direction of application of the magnetic field. Thus, the portions having the highest critical current density are adapted to transport the current in the high magnetic field, to regularly ensure high critical current density regardless of the direction of the applied magnetic field.
The inventive superconducting wire has no dependency on the direction of application of the magnetic field, but exhibits high critical current density with respect to any direction.
According to the inventive method, a superconducting wire exhibiting high critical current density can be produced through simple steps in high productivity.
The inventive oxide superconducting wire having the aforementioned advantages is usefully employed under a strong magnetic field particularly for a superconducting magnet having complicated field distribution.
According to one aspect of the present invention, the oxide superconducting layer provided around the central part is divided into a plurality of layers so as to increase boundaries between the oxide superconductor and a matrix as well as to increase pinning points, thereby improving pinning force. According to this aspect, the oxide superconducting layers are brought into configurations elongated along the circumferential direction as compared with the radial direction. In the working step, therefore, force is applied to the oxide superconducting layers perpendicularly to the circumferential direction, whereby the oxide superconducting layers are oriented along the longitudinal direction to facilitate flow of the current.
According to this aspect, preferably, high-resistance layers of a material having high electrical resistance are interposed between the plurality of oxide superconducting layers. When an alternating current is fed, a coupled current flows through a matrix layer provided between each adjacent pair of oxide superconducting layers. This coupled current results in a loss when carrying ac. In order to reduce such a loss, therefore, it is necessary to minimize the coupled current. According to this preferred embodiment, the high resistance layers of the material having high electrical resistance are interposed between the plurality of oxide superconducting layers to reduce the coupled current, thereby reducing the loss caused when carrying ac.
The material for the high-resistance layers may be prepared from a metal, ceramic or the like. When the matrix metal is prepared from silver, for example, a silver alloy or stainless steel may be employed as the material for the high-resistance layers. Such a material can also improve the strength of the matrix.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.